The invention relates to power switching semiconductor components and more particularly to triacs.
In many applications, it is desirable to control triacs by means of a diac placed upstream of the gate of the triac. A diac is a component with two terminals a little similar to an assembly of two diodes connected head to tail in series, or else to a transistor whose base is disconnected and not accessible, and it has a voltage-current characteristic with a very sharp threshold (acute angle bend in the characteristic) between a disabled region for voltages less in absolute value than a threshold voltage Vd, and a conduction region. The threshold is very sharp because the voltage-current characteristic has a negative dynamic resistance portion immediately beyond the threshold.
For example, a triac is often controlled by means of a diac for forming power variators or light dimmers.
In another application devised by the applicant, a diac is connected between the gate and the anode of a triac (triac with cathode gate), to obtain a component with two terminals only (anode and cathode of the triac) able to be connected as protective element against over voltages between two terminals of a circuit to be protected.
Faced with this requirement, efforts have been made to form the diac and the triac on the same semiconductor substrate. But the integration of these two elements raises problems. In fact, a triac is formed by a structure with four superimposed NPNP or PNPN layers, whereas a diac is formed by a structure with three superimposed PNP or NPN layers. Integration would therefore consist in using for the diac three layers from the four provided for the triac.
Unfortunately, this has proved very difficult for that implies that the main layer forming the semiconductor substrate then forms an active layer both for the diac and for the triac. It would then be necessary to isolate two different parts of the main layer, one for the diac, the other for the triac, which is not possible.
Other electrical or technological requirements make the integration of a triac and a diac very difficult; it is more particularly difficult to provide dopes and layer thicknesses which are compatible for the three layer structure of the diac and the four layer structure of the triac.